Page 17 - Gas Adsorption Equilibria
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             b)    adsorption isotherms for data correlation allowing to calculate gas-
                   adsorption-equilibria data at other gas concentrations, pressures and
                   temperatures.


             These data and correlation functions are needed in simulation programs to
          develop  and check  new or  better,  i. e.  smaller, faster  and  more energy-
          efficient adsorption based processes for a large variety of engineering, health
          and environmental purposes, cp. Sect. 2.

             In Sect. 3 the measurement methods for gas adsorption equilibria which
          are presented in  this  book are  outlined.  Several other  phenomena in gas
          adsorption systems  like the kinetics  of the  mass exchange process, which
          could not be considered here are mentioned in brief in Section 4. There also
          some general information  on  gas adsorption systems  will be  given and
          references for the various fields mentioned will be provided.



          2.       GAS ADSORPTION PROCESSES IN SEPARATION
                   TECHNOLOGY

             The sticking  of molecules  of gases or  liquids to the  surface of a solid
          material is called adsorption. It should not be mixed up with the phenomenon
          of absorption where molecules of gases or liquids are dissolved in another
          liquid or  solid material.  Adsorption  is  a surface  phenomenon which in
          principle  occurs at  any  pressure and  temperature.  Absorption is  a  bulk or
          volume phenomenon  which  may  or may not  occur at given  pressure  and
          temperature. The difference between both effects simply can be demonstrated
          by the sketch shown below. Here the cake symbolizes the molecule of the gas
          or liquid.  The person represents the solid material. Absorption means eating
          the cake. Adsorption occurs if the cake is splashed on the persons face.

             The interactions of a gas – normally a mixture – with the surface of a solid
          material can be fairly complex. This is due to the fact that the gas molecules
          can vary  considerably in  size, structure  and  electric  properties (dipole and
          quadrupole  moments), and  also the  surface of the  solid may  offer  different
          types of sites  for  adsorption,  reflected  in both the  pore  spectrum and the
          enthalpies of adsorption, cp. Chap. 1,  [0.4-0.6]. Hence one has to expect that
          interactions  between adsorbed  molecules of different type  will be  different
          from their possible interactions in a bulk gas or liquid phase.
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